1. Field of the Invention
This invention relates to an encoder arrangement for an apparatus having a rotatable control shaft carrying a rotating optical disc whose movements are sensed by photoelectric cell means whose signals are interpreted and applied to controlling the rotation of the shaft. This application is a continuation-in-part of application Ser. No. 25,583, filed Mar. 30, 1979.
The arrangement has specific utility in connection with the control of paper cutting apparatus, for example, as shown in application Ser. No. 25,583, the disclosure of which is hereby incorporated by reference.
2. Description of the Prior Art
As a basis for describing the present invention, the essential features of the paper cutting apparatus of the previous application will be described. The paper cutting machine has a table for supporting a pile of sheets to be cut, cutting means and means for operating it. A backstop is mounted for movement along the table so as to position and advance the pile. There is drive means for moving the backstop along the table in either direction including a worm shaft and signalling means made up as follows. An optical disc is mounted on the worm shaft for rotation with it. The disc is provided with a regular series of radial strips of predetermined width and spacing. A pair of photosensors is spaced-apart a predetermined distance about the disc whereby there are produced, as the disc rotates in either direction, wave forms corresponding to the strips. A controller is electrically connected to the photosensors and the apparatus includes computer mechanism which converts the wave forms into distances corresponding to the travel of the backstop. Preferably the signalling means is provided with a vernier arrangement which includes an additional marking on the disc and a photosensor for sensing that marking. As movement of the backgauge is initiated, the controller moves it back until it is stopped by its own control. The controller presets, roughly, the position of the backgauge. After this rough setting, the controller moves the backgauge forward until the photosensor senses the vernier marking and this signifies to the controller the exact position of the backgauge.